1. Field of the Invention
The present invention relates generally to systems for crossing voltage domains, and more specifically, to a fast converter for crossing voltage domains.
2. Background Art
In many electronic systems, multiple voltage supply domains are used. In general, digital electronic signals on an integrated circuit are transported using the lowest possible voltage, given the fabrication technology of the circuit. However, representations of the same signals are sometimes necessary in voltage domains of a higher supply voltage. In general, higher voltages are used in analog or mixed-signal circuits, or circuits interfacing signals to external circuits.
Referring now to FIG. 1, there is shown a block diagram illustrating the general problem of crossing voltage domains. In FIG. 1(a) a crossing from the high-voltage domain to the low-voltage domain is depicted. The actual signal crossing the domain boundaries is a voltage represented in the high-voltage domain. The problem is here to make the input of the circuit in the low-voltage domain in such a way that it can handle the signal represented in the high-voltage domain without being damaged.
In FIG. 1(b) a crossing from the low-voltage domain to the high-voltage domain is depicted. The actual signal crossing the domain boundaries here is represented in the low-voltage domain. Therefore, in general, the level of the signals from the low-voltage domain is not adequate to be received by the circuits in the high-voltage domain.
A generally known method used for crossing voltage domain boundaries is by first transforming the voltage domain signals to the current domain and then crossing the voltage domain boundary in the current domain. Referring now to FIG. 2, there is shown a circuit which illustrates this method for a high-voltage to low-voltage domain crossing in the current domain. In FIG. 2, the high-voltage devices are denoted by the thicker gates and the low-voltage devices are illustrated with the thinner gates. As is illustrated in FIG. 2, first the input signal is inverted, to enable the availability of both polarities of the signal. After inversion of the signal, two high-voltage NMOS devices transfer the signal to the current domain. The current is injected into a PMOS latch in the low-voltage domain. In this circuit, the high-voltage NMOS devices are only able to pull down the nodes of the latch, while the latch itself can only pull up its two nodes.
Referring now to FIG. 3, there is shown a prior art circuit for a low-voltage to high-voltage domain boundary crossing. The circuit in FIG. 3 is similar to the circuit 200 in FIG. 2 when high and low-voltage devices are swapped. However, the two pull-down NMOS devices still remain high-voltage devices. At the gates of these two high-voltage devices however, a signal coming from the low-voltage domain is used which will make the response of these devices very slow. In many circuits, the inherent low speed capabilities of this circuit are a problem.
Therefore, what is needed is a circuit for crossing voltage domains without the problems in the prior art.